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Recent refinements in MEMS processing have resulted in the batch fabrication of low cost, high performance, miniaturized condenser microphones. In certain applications, this device offers specific advantages over traditional ECMs. Starting with a silicon wafer, semiconductor materials are deposited and removed to form a capacitor. Just as with a conventional electret microphone, it consists of a flexible diaphragm, a stiff backplate and damping holes with an electrical charge on the backplate. The diaphragm is in close proximity to the backplate, forming a capacitor. When sound pressures impinge on thediaphragm, it moves, changing the capacitance between the plates. This variance is measured and out-putted as an electrical signal.

Apart from being made of silicon, the largest difference between an ECM and a silicon microphone involves how the charge is maintained on the backplate. A silicon microphone does not have a charge when it leaves the factory. A charge is “pumped” onto the backplate via a CMOS circuit. The chip maintains this charge whenever the microphone is activated.

Traditional electret microphones utilized in portable wireless applications achieve high sensitivity through compliant diaphragms that are large in size (typically 6 mm or 4mm). As ECMs are reduced in size, they quickly lose sensitivity. The Knowles Acoustics surface mount microphone maintains a high sensitivity (-42 dBV); even though the diaphragm is just 0.5 millimeters in diameter. This is accomplished using a patented free-floating diaphragm. The small size of the diaphragm results in significant economies of scale and low cost, since thousands of microphones can be made from a single silicon wafer.

From:http://www.knowles.com/index.htm